As MagV mentioned, Typed Scheme (http://www.ccs.neu.edu/home/samth/typed-scheme/) is an example of static typing in a lisp-like language. I've used it, and it is great. It interoperates smoothly with the normal dynamic typed Scheme. Typing is at the module level (a module must be either statically or dynamically typed) and contracts (http://doc.plt-scheme.org/reference/contracts.html) are used to enforce invariants at module boundaries.
If you want static typing, then I don't think trading out dynamic typing is a big deal to you.
Though as pointed out else where, many implementations allow you to give their compiler type hints... which is a sort of half-way "best of both worlds" system.
The halfway "best of both worlds" is impossible to attain, in my opinion. As far as I understand it, dynamic typing (meaning, extensive usage of runtime type information) has unmatched flexibility. Static typing (meaning, extensive analysis of syntactic types at compile time) completely prevent large classes of errors.
If you want a middle ground, you may lose some of the flexibility, and you still won't be able to prove as much as a full static type system. In the end, the "best of both world" could rapidly become the worst of both worlds.
As I see it, we have to compromise. When you design a type system, you want to maximize 3 virtues: flexibility, simplicity, and error sensitiveness. Alas, of these 3, you can only have 2. Dynamic type systems typically are simple and flexible, but hardly prove anything (which explain why unit tests are so useful). Advanced type systems like Haskell's are quite flexible and prevent many errors, but they are complex. Others, like Java's, are simpler but not as flexible (nor as error proof). And of course you have horrible type systems, like C++'s, which lacks all 3 virtues.
I find that with a good, type-aware compiler such as SBCL, Common Lisp hits a really sweet spot on the typing issue. It catches a crapload of mistakes at compile time that would be runtime errors in Python (the language, incidentally the SBCL compiler is also named Python) and most other dynamic languages. The type-language of CL is also more expressive than anything else I have seen, featuring unions, intersections, predicates, subtyping etc. But most importantly, it stands in the background, and doesn't stop you from executing programs that don't conform to some preconceived notion of what typing should be like.
On the other hand you could make an argument that static typing isn't actually a language feature as much as it is a step in the process of programming.
Static typing consists of (a) tagging variables with their type and (b) running a program that uses these type tags to check and/or rewrite the program.
It's easy to add type tags to a lisp program. It's just that Lisp doesn't specify that second program that checks and transforms the first. So I would say Lisp is half way there when it comes to static typing.
no, adding type declarations to Lisp is the easy part. when it comes to static typing, standard Common Lisp offers very little.
Declaring types? That has been done. In Common Lisp:
(defun twice (n)
(declare (number n))
(the number (* n 2)))
The difficult parts are:
* the type system and its capabilities
* make the operations of the type system sound
* determining sub-types
* type inference
* integration with the rest of the language (where data objects also have something like types)
Common Lisp provides lots of infrastructure for all kinds of things, but very little for a type system. For example in Lisp one can determine the value of an expression via EVAL, but there is no function to compute the type of an expression (other than a type of the computed value).
